Well, I would say there is a good chance that a single person crying on their own is not in much immediate danger, and possibly is going to head to a place of comfort. They might not be, or they might be lost, or something, or they might be really emotionally distressed. I reckon I would be prepared for a situation where someone was lost or whatever, but I think I am unequipped to help out with more complex problems if I don't know the person. So, for a single person, it's probably okay to offer directions or some comfort, but you probably can't provide much else, especially in the short run. If they don't like you asking, not much will happen, they'll still be sad but probably not sadder.
With two people, though ... I remember in Melbourne a guy was dragging a woman into a cab by her hair and someone intervened - and that person got shot and killed. So it is definitely worth assessing the safety of the situation first, and weighing up your abilities against probable reactions.